The difference is that some skills bound to LMB allow you to use the default attack when they are on cooldown instead of not moving and saying "the skill is not ready yet." They allow for interesting caster/melee hybrid builds.

Except you could just do that manually. You don't actually need LMB for that to be a viable build. As I said, I'm all for everything being bindable everywhere...Just not for that, specific, reason. Those caster/melee hybrids are already there, just not as convenient to play.

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For those who need comprehension aids:
Best/Opinion/Fact http://www.grimdawn.com/forums/showthread.php?p=492058#post492058

Skills that target the ground will never be on LMB as they severely impact movement (in before "just bind Move To on another key", but that's not ideal nor should it be a requirement for a control configuration to function).